Wilting that persists despite adequate soil moisture may indicate a disease that affects the roots or vascular system of the plant.  Black shank is a root rot disease that usually results in wilting of the whole plant, rather than one side of the plant.  Wilting and yellowing often begin on the lower leaves.  This video shows black shank symptoms at various stages of growth.

Rotting of the roots decreases uptake of water and nutrients, resulting in above ground wilting and yellowing symptoms.  Wilted plants may or may not also have blackening of the stem tissue.  Infected plants may lose almost all of their root systems to rot.  This video shows different patterns of root rot in black shank-infected plants.